Creep-resistant lead base alloys



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CREEP-RESISTANT LEAD BASE ALLOYS Filed May 1. 1946 4 Sheets-Sheet 4 BY SHMMK ATTORNEYS Patented @ch igi UNETED CREEP-RESISTANT LEAB BASE ALLOYS Application May 1, 1946, Serial No. 666,461

4 Claims.

This invention relates to lead base alloys, and ls especially concerned with increasing the resistance to creep and otherwise improving the physical properties of lead base alloys comprising arsenic. The invention provides for accomplishing such improvements by incorporating a small amount of tin, cadmium or zinc in the alloy, and heat-treating the resulting alloy. The method of the invention isapplicable to lead base alloys containing, in addition to a small amount oi' arsenic, small amounts of other metals such as bismuth. The improved lead base alloy of the invention is especially valuable for use as a cable sheath. This application is a continuation-inpart of my prior application Serial No. 537,504, filed May 26, 1944, now abandoned.

Many electric cables of the lead-sheathed type contain oil as an insulating medium. In some such cables a channel is formed to permit the oil to flow freely in the cable, Whereas in others the oil is impregnated into a solid but permeable insulation such as paper and so is relatively less free to ow in the cable. In all such cables, however, the oil can flow to some extent, and consequently when such cables are run vertically or at an angle to the horizontal, a hydrostatic pressure of oil develops in the lower part of the cable.

. 'I'his pressure must be resisted by the lead or lead alloy sheath. Commercially pure lead and lead alloys heretofore available possess rather low resistance to creep and rather low general endurance under prolonged stress, and have not proven very effective in resisting the hydrostatic pressure of the oil in cables, especially in installations where such pressures are of considerable magnitude. On the contrary, cable sheaths of lead and lead alloys formerly available have been subject in greater or lesser degrees to distension under prolonged, steadily exerted hydrostatic oil pressures, and many cases are known where the lead sheath ultimately has burst, ruining the cable. With some types of lead base alloys used -as cable sheaths, the distension that occurs in consequence of steady hydrostatic pressure is not great but a brittle-type failure akin to stress .corrosion cracking occurs at an early date. In modern power cables, the insulation has been so improved that the component limiting the life of a cable is in fact the lead or lead alloy sheath. A

Binary alloys of lead and arsenic, containing arsenic in amounts of the order of 1% or less,

have been known ior many years, and in investigations of the constitution diagram and properties of such binary alloys, it has been found that these alloys harden when treated by heating to a temperature approaching the melting point and then quenching. These same investigations have revealed, however, that the alloys hardened by this heat treatment procedure are subject lto rapid self-annealing at room temperature, resulting in reversion of the properties of the alloy to the original values prior to heat treatment.

Bauer and Tonn (Zeitsch. f. Metallkunde, vol. 27, pages 183-187) attribute hardening of leadarsenic alloys by heat treatment to the formation of a solid solution of arsenic in the lead, and they attribute the self-annealing of these alloys at room temperature to the reprecipitation of arsenic from the solid solution. This in effect indicates that the recrystallization temperature of heat treated binary lead-arsenic alloys is not above about room temperature. Bauer and Tonn conclude from their investigations that the addition of arsenic alone to lead is not capable of producing sucient improvement in the mechanical properties of lead to make these binary alloys more useful than lead and other known lead alloys. My own investigations confirm Bauer and Tonn in this respect, in that I have found that the rapidXself-annealing of lead-arsenic binary alloys makesit impossible to take advantage of the improved properties developed by heat treatment in any article in which such improved properties should be preserved over a substantial length of time, as, for example, in cable sheaths. Johnson and Bassett (U. S. Patent No. 2,300,788) have proposed ternary alloys of lead, arsenic and bismuth for use as cable sheaths (they found that the addition of bismuth improved the ductility of the alloy). but I have found that these alloys too are subject to selfannealing at room temperature, and particularly are subject to loss of bursting strength in the case of cable sheaths made therefrom (especially if the alloy is heated to temperatures ofA the order of 50 C. to r200 C.). Cable sheaths may become heated to the lower temperatures within this range during heavy service, and quite commonly are subjected to the higher temperatures in this range when separate lengths are joined together. Joints usually are made by heating the sheath and joining by the well-known method of producing wiped lead joints. Such-heating of a sheath of a ternary lead alloy containing arsenic and bismuth results in a relatively soft section of relatively low creep resistance and low bursting strength adjacent to the joint.

In contrast to the property of undergoing selfannealing is the property of age-hardening, to which a number of lead base alloys are subject. For example, lead base alloys containing antimony and lead base alloys containing calcium age-harden considerably (the former quite rapidly and the latter more slowly). Whereas selfannealing of heat-treated lead-arsenic alloys reduces bursting strength of cable sheath made therefrom by resulting in a loss of the physical properties developed by the heat treatment (including loss in resistance to creep), age-hardening of lead base alloys results in a loss of bursting strength of cable sheath made therefrom in consequence of the fact that the age-hardening is accompanied by a loss in ductility of the alloy. This results in a reduction in the fatigue resistance and general endurance of the alloy, as indicated by the fact that it cracks more readily under repeated lexure or under sustained stress. Since power cables in service are subject to thermal expansion and contraction with daily variations in the cycle of the power load carried by the cable, they are subject to repeated ilexure and variations in pressure, and the lead sheath therefore should possess high endurance, including high fatigue resistance. Age-hardening of a lead base alloy, by reducing its endurance, impairs the value of the alloy for use as a cable sheath for long-time service. Cable sheaths composed of age-hardening alloys, when maintained under moderate stress, usually fail by cracking of the type known asV stress corrosion cracking long before they have stretched or crept to the extend indicated by their original ductility. (The term endurance is used herein to signify resistance to failure both of the stress corrosion cracking type and of the fatigue type.)

I have discovered that if a small percentage of tin, cadmium or zinc is incorporated in lead base alloys containing arsenic, with or without minor added quantities of metals, such as bismuth, which is a constituent of the alloy of Patent No. 2,375,755, grantedMay 15, 1945 on an application of William H. Bassett, Jr. and mine, the annealing temperature of the heated and quenched alloy is considerably increased and the creep resistance of the alloy is very greatly increased. Both of these properties are in fact increased to such an extent that wiped joints may be made in a cable sheath made of such modiiied alloys without unduly softening the sheath or lessening its creep resistance or bursting strength adjacent the joints. I have further discovered that lead base alloys containing arsenic (with or without additions such as bismuth) in which the above-indicated quantity of tin, cadmium or zinc has been incorporated, do not age-harden significantly, and that these alloys possess a high endurance (resistance to stress corrosion cracking and fatigue types of failure) throughout a prolonged period of time.

Based on this discovery, my invention provides the method of increasing the endurance and resistance to creep of a lead base alloy containing about 0.02% to 1% arsenic, which comprises incorporating in the alloy about 0.01% to 1% of a metal of the group consisting of tin, cadmium and zinc, heating the resulting alloy to an elevated temperature, and then quenching the heated-alloy. Heat treatment temperatures.

4 between 190 C. and the melting point of the alloy, followed by quenching in water, are generally effective. In the heat treatment of previously fabricated articles (such as extruded cable joint sleeves), a temperature in the range 250 C. to 290 C. is generally best; while in the manufacture of cables involving extruding the sheath about a cable in a lead press in which the lead is melted and is extruded without intermediate cooling to room temperature, an extrusion temperature in the range 200 C. to 235 C. is very satisfactory. Quenching in water generally is preferable, but if desired, the quenching may be in some other medium such as oil or an air blast.

The improved physical properties of the heattreated alloy appear to be due to the fact that the heating step produces a solid solution of arsenic in lead, and the quenching retains the arsenic in the solid solution. Accordingly, the heating should be at a temperature suiliciently high and for a time suillciently long to put a substantial part of the arsenic in solid solution, and the quenching should be rapid enough and should follow the heating step sumciently promptly to insure retaining the arsenic in solid solution. The tin, cadmium or zinc incorporated in the alloy appears to have the effect of preventing substantial reprecipitation of the arsenic from the solid solution at room temperatures or at mildly elevated temperatures (up to 50 C. or even C. for short periods of time), and one of these elements therefore is an essential constituent of the alloy in order to insure retention over a prolonged period of the improved physical properties developed by the heat treatment.

The above-described treatment results in the provision of a heated and quenched lead base alloy comprising 0.02% to 1% arsenic and the balance chiey lead, which is characterized by the fact that the alloy contains 0.01% to 1% of a metal of the group Aconsisting of tin, cadmium and zinc, is highly resistant to creeprover a prolonged period of time at room temperature and even up to 50 C., and does not age-harden signiiicantly. 'I'he indicated creep rate of the improved alloy, when prepared and heat treated under optimum conditions, is less than about 0.05% per year when subjected to a stress of 250 pounds per square inch at room temperature, and the alloy possesses a very high endurance (high resistance to fatigue failure when subjected to repeated flexure and high resistance to stress corrosion cracking when subjected to sustained stress).

'I'he invention is particularly applicable to the manufacture of cables, especially oil-lled or oilimpregnated power cables and other types of cables subjected to substantial internal pressure, which are encased in a lead sheath. 'I'he invention, therefore, contemplates imparting high endurance to a lead base alloy cable sheath containing about 0.02% to 1% arsenic and making it highly resistant to creep over a prolonged period, which comprises incorporating about 0.01 to 1% of a metal selected from the group consisting of tin, cadmium and zinc in the alloy, extruding the resulting alloy as a sheath about the cable at a temperature in the range from 200 C. to 235 C., and quenching the sheath immediately as it emerges from the die to cool it rapidly. The same method is, of course, applicable to the manufacture of other extruded articles, such, for example, as sleeves for use in joining lengths of oilfilled or oil-impregnated power cable. While the above-mentioned extrusion temperature of about 200 to 235 C. is generally optimum for treating an extruded sheath on a cable, eiective results may be obtained at extrusion temperatures in the of the alloy.

The improved properties of the alloys prepared in accordance with the invention are indicated in Table I. The several alloyslisted in Table I contained the various ailoying elements in the percentages given, the balance of the alloy in each case being substantially all lead. The figures given in the table denote hardness of the alloy as determined on the Rockwell scale, using a one-half inch ball under a sixty kilogram load for thirty seconds. The letters C" and F" designate respectively cast test specimens and forged ,test specimens. The figures in the columns headed Original give the hardness of the cast and forged specimens before heat treatment. figures in the columns headed Annealed give the hardness of the test specimens after heating for nineteeen hours at 100 C. and then cooling slowly in air. The figures in the columns headed Hardened give the hardness of the test specimens after heating at the temperature and for the time indicated in the footnote and then quenching in water. The letter following each hardness number in the columns headed "Hardened refers to the particular footnote giving the heat treatment.

The 20 strength better after being subjected to annealing temperatures than do the alloys containing none of these additions. 5

Table I also shows the susceptibility of arsenicwider range between 190 C. and the melting point 5 bearing lead alloys-generally to be hardened or softened by appropriate heat treatment. As indicated in the table, the alloys may be hardened appreciably by heating at anv elevated temperature upwards from 190 C., preferably for an hour or more. and then quenching. Quenching most advantageously is accomplished with water, but other quenching media such as oil or a blast of air may be employed.

Table I also shows that the alloys not containing tin, cadmium or zinc (particularly those not containing tin or cadmium) are softened appreciably by heating at a relatively low temperature and then cooling slowly. The alloys containing tin or cadmium, and to some extent the alloys containing zinc, do not soften very much at the lower annealing temperatures in the neighborhood of 100 C. This is indicative of the fact that the physical properties developed by heat treatment, including high bursting strength, are retained in these alloys even after such heating as occurs in the vicinity of wiped joints of a cable sheath.

The response of arsenic-bearing lead alloys to this type of heat treatment is unusual for nonferrous alloys. Such alloys generally are softened by heating to a relatively high temperature Table I Original Annealed Hardened Per P" other comi Cent Cent Ponent l As B o F. o F o F.

0. 15 82 106 58 73 1340 143|! 0. 15 0. 17 109 119 66 79 1400 1460 0. l5 0. 17 0 05% Sn 111 130 134 123 151e 1424i 0. l5 0. 17 0 15% Sn 114 141 142 138 158C 1554 0.15 0. 17 0 25% SI1 117 149 140 139 150e 144e 0.15 0. 17 0 50% Sn 125 145 139 104 144C 119e 0.15 0. 17 l 0% Sn 129 126 121 140C 127e 0. 15 0. 17 0 01% Cd 103 139 102 1m 1231! 1290 0. 15 0. 17 0 10% Cd 109 155 133 107 13241 13011 0. 15 0. 17 0 15% 118 148 137 111 13la 12841 0. 15 0. 17 0 25% Cd 127 140 142 129 1350 13841 0. 15 0. 17 0 50% d 112 132 111 97 11741 12Go 0. 15 0. 17 0 01'7 Zn 108 121 106 88 1460 1490 0. l5 0. 17 0 05 a Zn 113 146 103 1480 1580 0. 15 0. 17 0 10% Zn 108 148 97 82 152') 1540 0. 15 0. 17 0. 25% Zn 100 149 Y 102 89 1500 1540 0.16 0. 17 0. 50% Zn 112 149 78 y 93 1410 1530 a. Hardened by heating 4 hours at 205 C. and quenching in water. b. Hardened by heating 6 hours at 260 C. and quenching in water. c. Hardened by heating 4 hours at 260 C. and quenching in water. d. Hardened by heating 24 hours at 260 C. and quenching in water.

It will be noted from Table I that lead alloyed with arsenic alone, or witharsenic and bismuth without other additions, becomes appreciably softened by the annealing procedure (at only 100 C.) The alloys containing tin or cadmium in addition to arsenic and bismuth, on the other hand, softened but very little when subjected to the same annealing procedure. The alloys containing zinc in addition to arsenic and bismuth softened to some extent during annealing, but the 1 cast samples p-articularly were definitely superior in this respect to the alloys of lead with arsenic alone or with only arsenic and bismuth.

Hardness by itself is not a measure of' the bursting strength of a lead alloy formed into `a cable sheath. However. alloys having very good bursting strengths in the heat-treated condition suiier a decrease in bursting strength concuror zinc have been added retain their bursting and cooling rapidly, and are age-hardened by reheating to a relatively low temperature for a considerable period of time. g.

The susceptibility of bismuth-free arsenicbearing lead alloys` to softening and hardening at various heat treatment temperatures is indicated in Tables l1 and III. Table II gives the hardness on the Rockwell scale (determined by using a one-half inch ball under' a sixty kilogram load for thirty seconds) of cast and forged test specimens of a binary arsenic-lead alloy and ternary arsenic-tin and arsenic-cadmium lead alloys after annealing at the temperatures indicated for twenty-four hours and then cooling slowly. Table III gives the Rockwell hardness (determined in the same manner) of cast and forged test specimens of the same alloys after hardening by heating for six hours at the temperature indicated and quenching in water. In each of Tables II and III, the letters C and and forged test specimens.

Table 1I Heated 24 hours at temperature indicated and slowly cooled Per Cent Per Cent Per Cent Original Aa Sn Cd Sample 50 o 05 o 80 o 90. C. 100" o. 150 o. 200 0.

C. 125 109 B7 89 67 67 73 80 -15 F. 129 119 102 a0 e1I` 15 19 sa o 25 {C. 14s 139 139 133 11s 124 121 101 0 15 F. 142 144 139 121 111 102 10a 90 o 25 {(1. 14a 123 129 110 110 101 117 105 0-15 F. 14s 130 119 111 111 101 95 101 Table III Heated 6 hours at temperature indicated and quenched in water Per Cent Per Cent Per Cent Original AB Sn Cd Sample 121 o. 149 c. 111 C. 204 C. 212 C. 260 c. 289 o.

{(1. 125 100 15 s4 94 124 12a 15s 0- 15 F. 129 111 s4 85 100 112 133 165 0 25 {C. 146 139 134 139 130 148 145 105 F. 142 133 10s 110 113 12s 139 159 l 0 25 {o. 143 112 112 105 130 132 151 110 0- 5 F. 14s 111 100 11s 124 1 134 145 1ra Table 1I shows that the binary arsenic-lead alloy becomes rapidly annealed, with substantial loss of hardness at temperatures even as low as 50 C. This loss of hardness at low annealing temperatures is indicative of diminished values of physical properties, including resistance to creep, which are associated with bursting' strength of cable sheaths composed of the alloy. As further shown in Table 1I, the addition of tin or cadmium to the alloy raises the annealing temperature so that it does not soften very much at temperatures below 100 C. The results set forth in this table are indicative of the manner in which the properties of the tin-bearing and cadmiumbearing alloys associated with high bursting strength of cable sheaths are retained even at temperatures up to 100 C.

Table III shows that alloys not containing bismuth are amenable to development by heattreatment of hardness and other properties associated with high bursting strength of cable sheaths.

The charts reproduced in the accompanying .drawings further illustrate the advantageous physical properties,l especially useful in cable sheath alloys, developed and retained in lead base alloys prepared in accordance with the present invention. The nominal composition of the new arsenic-tin-lead alloyfor which data are plotted in the accompanying drawings was in all instances 0.15% arsenic, 0.10% tin, 0.10% bismuth, and the balance lead except for commercial impurities (this has proved .to be a particularly satisfactory alloy composition). For comparative purposes, the chartaalso include curves showing the properties of commercial lead and heretofore known lead alloys of the types used in making cable sheaths. Y

Fig. l is a plot of the tensile strength of lead base alloys against age of the alloys. The curve for the arsenic-lead-tin alloy prepared and heat treated in accordance with the invention shows an initial small drop in tensile strength after which the tensile strength remains essentially constant throughout a period of years. This is opposite to the behavior of antimony-lead, which is seen to age-harden rapidly during the r'st few months and then lose tensile strength with the, passage of time. Calcium-bearing lead agehardens steadily, indicating that with the passage of time its endurance diminishes and it becomes stress corrosion cracking. Copper-bearing lead retains its tensile strength at a substantially constant value over a period of years, but its strength is considerably below that of the new alloy.

Fig. 2 is a plot contrasting the high fatigue resistance of the new alloy with that of ordinary commercial lead for power cable sheaths. It will be noted that the long-time ('10 minute) bending cycle causes failure more rapidly than a shorttime bending cycle. Cable sheaths in service are subject to bending cycles of long-time duration, and failure may therefore be expected after a fewer number of cycles than would be indicated by high speed fatigue tests. 'I'he curves show that the new alloy possesses a much higher fatigue strength, especially when subjected to long-time bending cycles, than ordinary cable-sheath lead subjected to the same cycle of bending stresses.

Fig. 3 shows the high creep resistance of the new alloy compared with ordinary commercial cable-sheath lead. In this figure the indicated creep rate at several temperatures in percent per year, as determined by the extent of creep that occurred during the 1000-hour interval between 1000 hours and 2000 hours of applied stress, is plotted against the applied stress. Creep measurements were made for the time interval between 1000 hours to 2000 hours to eliminate the effect of erratic results obtained in many'alloys during the "initial period of creep tests, particularly at low applied stresses. Fig. 3 shows that at room temperature and at 43 C., the new alloy possesses an indicated creep rate that is much lower than ordinary cable lead, and even at 66g C., the indicated creep rate of the new alloy is only slightly higher than that of ordinary lead at room temperature. For example, at 250 pounds per square inch applied stress, the new alloy at room temperature has an indicated creep rate less than 0.05% per year, and at 43 C., the indicated creep rate is less than about 0.15% per year. Even at 66 C., the indicated creep rate is only about 0.7% per year. On the other hand, commercial cable lead under the same conditions possesses an indicated creep rate at room temperature of almost 0.5% per year and its indicated creep resistance decreases with temperature so that at 43 C. it amounts to over 1% per year.

(The term indicated creep rate is used here- \ln to denote the creep rate in percent per year asrdetermined from short-time tests. It thereincreasingly susceptible to fatigue failure and fore refers to an extrapolated value (when ex..

pressed in terms of percent per year) and is not to be taken as meaning that in a test lasting for a year all alloys will creep without failure to the full extent of the indicated creep rate. Particularly at high applied stresses, failure occurs considerably before a year has elapsed. Under such conditions the indicated creep rate, expressed in percent elongation per year. may be quite high even for age-hardening alloys. When these latter alloys are subjected to a low applied stress, a brittle failure of the stress corrosion cracking type occurs long before the alloy has crept to the extent which would be expected on the basis of indicated creep rate and percent elongation values determined from short-time high-stress tests.)

Fig. 4 further shows the high resistance to creep of alloys prepared in accordance with the invention. In this figure, the creep was measured by applying hydraulic pressure to the interior of tubes of approximately the dimension employed on average size power cables. The new heattreated alloy is seen to possess the smallest percentage increase in diameter at all times up to two years of any of the considerable number of alloys represented in this figure. Calcium-lead containing 0.025% calcium is seen to possess the next lowest creep rate (percentage increase in diameter), but it will be observed that the tube composed of this alloy was subjected to a considerably lower hoop stress (535 pounds per square inch for the 0.025% calcium-lead as compared with 591 pounds per square inch for the new alloy). All other alloys show substantially higher creep rates. The curve for the non-heat-treated arsenic-tin-lead alloy shows that without heat treatment the creep rate o this alloy is very high.

Fig. 5 shows the bursting strength of cable sheath tubes made of the new alloy, in comparison with other cable sheath lead alloys, under various applied hoop stresses. This figure is perhaps the most signiiicant of those presented in the accompanying drawings, insofar as it bears on suitability of the alloy for cable sheath use. An interesting and signicant fact is that all heretofore known lead base cable sheath alloys have a life-before failure of about two years when subjected to a hoop stress of about 500 pounds per square inch, even though they possess widely diiering lives to failure at higher hoop stresses. The alloy prepared in accordance with the invention is unique in that its bursting strength does not follow this general trend, but is instead substantially higher at the lower hoop stresses commonly encountered in oil-.filled and oil-impregnated cables. higher burstingv strength at very high hoop stresses, but the bursting strength of this alloy falls oi to below that of the new alloy at hoop stresses below 1500 pounds per square inch. Other lead base cable sheath alloys possess very -much lower bursting strengths than the new alloy, at equal hoop stresses.

The data presented graphically in the accompanying' drawings, insofar as it applies to the new alloy, are for lead base alloys containing 0.02% to 1% arsenic and 0.01% to1% tin, after having Calcium-bearing lead possesses a- Si li 1 been heat treated as herein described. The high strength physical properties of the alloy are not achieved except after heat treatment, and these properties are not retained for a long period of time except in the presence of tin,- cadmium or zinc.

I claim:

1. A lead base alloy composed of 0.02% to 1% arsenic, 0.01% to 1% of a metal of the group consisting of tin, cadmium and zinc, and the balance lead, characterized in that the arsenic is present in solid solution in the lead, said alloy having an indicated creep rate less than about 0.05% per year when subjected to a stress of 250 pounds per square inch at room temperature and being insensitive to age-hardening.

2. A lead base alloy composed of 0.02% to 1% arsenic, 0.01% to 1% tin, and the balance lead, characterized in that the arsenic is present in solid solution in the lead, said alloy having an indicated creep rate less than about 0.05% per year 4when subjected to a stress of 250 pounds per square inch at room temperature and being insensitive to age-hardening.

3. A cable having a sheath composed of 0.02% to 1% arsenic, 0.01% to 1% of a metal of the group consisting of tin, cadmium and zinc, and the balance lead, characterized in that the arsenic is present in solid solution in the lead, said sheath having an indicated creep rate less than about 0.05% per year when subjected to a stress of 250 pounds per square inch at room temperature and. being insensitive to agehardening.

4. A cable having a sheath composed of 0.02% to 1% arsenic, 0.01% to 1% tin, and the balance lead, characterized in that the arsenic is present in solid solution in the lead, said sheath having an indicated creep rate less than about 0.05% per year when subjected to a. stress of 250 pounds per square inch at room temperature and being insensitive to age-hardening.

CLERMONT J. SNYDEB.

REFERENCES CITED The following references are of record in the file of this patent:

' UNITED s'ra'rss PATENTS Date Number Name 1,890,014 Dean Dec. 6, 1932 1,896,473 Townsend Feb. 7, 1933 2,163,369 Better-ton June 20, 1939 2,277,627 Bouton Mar. 24, 1942 2,375,755 Bassett. Jr., et ai. May 15, 1945 FOREIGN PATENTS Number Country Date 201,176 Great Britain Feb. 21, 1924 OTHERREFERENCES` stracted in Chemical Abstracts, vol. 70, columns 2154 and 2155. 

1. A LEAD BADE ALLOY COMPOSED OF 0.02% TO 1% ARSENIC, 0.01% TO 1% OF A METAL OF THE GROUP CONSISTING OF TIN, CADMIUM AND ZINC, AND THE BALANCE LEAD , CHARACTERIZED IN THAT THE ARSENIC IS PRESENT IN SOLID SOLUTION IN THE LEAD, SAID ALLOY HAV- 